CHARACTERIZATION AND LOCALIZATION OF A PHENOLOXIDASE IN MUNG BEAN HYPOCOTYL CELL-WALLS

The occurrence of proteins able to oxidize polyphenols even in the abs ence of H2O2 was recently reported in mung bean (Vigna radiata L.) hyp ocotyl cell wall extracts (R. Goldberg, A. Chabanet, A.M. Catesson [19 93] In K.G. Welinder, S.K. Rasmussen, C. Penel, H. Greppin, eds, Plant Peroxidases: Biochemistry and Physiology, pp. 296-300). Therefore, th e possible presence of a laccase in the extracts was investigated usin g immunocytological and biochemical approaches. An enzyme catalyzing p henol oxidation in the presence of molecular O-2 was extracted and pur ified from the cell walls. This 38-kD cationic protein, like o-dipheno loxidases, was unable to oxidize p-diphenols or p-diamines. However, i t crossreacted with an anti-laccase antiserum and, like laccases, its activity was inhibited by N-cetyl-N,N,N-trimethylammonium bromide but not by ferulic acid salts. Immunolabeling data showed that the 38-kD o xidase was absent from all cellulosic cell walls. It was localized onl y in lignifying and lignified cell walls. This restricted localization suggests that this laccase-like phenoloxidase could participate in th e lignification process but not in the primary wall stiffening, which develops in the epidermal and cortical tissues along the mung bean hyp ocotyl.